High purity electrical power generally means that the power is free from voltage spikes and sags with zero neutral-ground voltage. A number of electronic devices require such high purity power. Among them, in particular, are medical imaging systems such as X-ray, computerized tomography, magnetic resonance imaging, and radiation treatment systems. All of these systems require a large amount of current but only for a short duration when the X-ray or magnetic generator is operational. The power during this exposure must be clean for good image quality. Additionally, the stand-by power between exposures must also be clean for the reliable operation of the computerized control and image processing subsystems which operate between exposures.
For these types of systems, it is also very important that the voltage drop during exposure be minimal, typically less than eight percent. This includes the impedance of all upstream wiring, connections and transformers. The reason is that the exposure duration is calculated on line voltage present immediately before exposure. Significant changes during exposure could result in unpredictable dosages. It is also important that operation of the generator does not create voltage sags or spikes which interfere with the reliable operation of other system components.
The National Electric Code (NEC) requires that circuit breakers feeding the circuit powering an X-ray system be sized for at least 50% of the maximum current draw of the system. This forces the use of larger building wiring which achieves lower impedance and reduces the possibility that the breaker could fuse closed.
Various methods have been employed to satisfy the requirements of the electronic equipment mentioned above. The most common method to power these systems is to run large size, dedicated wiring from the building service entrance. In an effort to minimize neutral-ground voltages, this wiring often has a full-size neutral and ground. Since this is a computer-type load, it is possible that there could be requirements to oversize the neutral compared with phase conductors. There is little effective isolation from the rest of the building and the power quality that results with this "solution" is low.
Another way to achieve the desired power quality is to use surge suppression devices or L-C filters, or both, on the building wiring near the load. These devices shunt impulses above certain voltage or frequency levels from one wire to another. They typically comprise metal oxide varistors (MOV's), silicon avalanche diodes, gas discharge tubes, capacitors and inductors, and often incorporate resistors.
There are several limitations with these types of devices. One is that their effects are limited since they can only protect to a certain voltage or frequency level. MOV's and avalanche diodes "wear out" with time and lose their effectiveness. To the degree that these devices and filters shunt away voltage spikes or dips ("normal mode noise"), they dump them onto the neutral conductor, creating neutral-ground potentials ("common mode noise") which are even more damaging or disruptive than normal mode noise.
To be effective, the inductance between the power line and the shunt elements (surge suppressors and capacitors in L-C filters) must be minimized. Each foot of wire length connecting the "suppressors" to the conductors makes a measurable difference in their effectiveness. Often these devices are connected to the power lines by wires which are 5 to 50 feet in length due to physical placement constraints in the field or limited knowledge on the part of the installers, or both.
Another alternative solution is to use a conventional shielded isolation transformer. The transformer allows a new neutral to be derived on its load side, which means that the input ground can be reduced in size to code minimums, and no input neutral needs to be run at all to the transformer. The shield in the transformer increases the isolation of the output from the conducted ground (common mode) noise. The fresh neutral-ground bond converts common mode noise to normal mode noise and allows a more effective use of filters and surge suppressors on the transformer secondary as described above. A disadvantage of conventional transformers is that the added impedance of the transformer means that input wiring needs to be increased in size so that total impedance to the load would still be low enough. For that reason, as well as the NEC limitation on the minimum size for main breakers, this isolation transformer needs to be sized for at least 50% of the momentary load, and typically 70-100%. That means it sits idling at about 10% or less of its rated power, wasting money, space and generating objectionable heat and noise which prevent it from being located near the electronic equipment which it powers.
The extra impedance of the shielded isolation transformer also results in lower power quality when it interacts with computer loads. Modern computers have "switch mode" power supplies which draw their current in short bursts where the change in current with respect to time (is/dT) is fast, being equivalent to 1 KHz instead of the usual 60 Hz for most conventional loads. Even at low load factors, conventional transformers have outputs with a flat-topped voltage waveform and have voltage spikes. Worse, when one of the many system loads switch on or off an even higher effective frequency is generated which results in even larger voltage spikes.
There are times that shielded isolation transformers and suppression/filtering devices are combined in the field in an attempt to provide the quality power desired. The limitations mentioned above apply to this combination.
Conventional voltage regulation devices cannot be used. Electronic-controlled tap-switching voltage regulators are undesirable because tap changes during exposure cannot be tolerated. Saturable-core ferroresonant transformers have very high impedance and slow reaction time. They interact with the pulsed load by creating large voltage transients.